It isn't about him being sexist, racist, ignorant, dishonest or offensive - you know, any of the usual stuff.

This time it is more serious. Mr Trump didn't just suck up to a murderous dictator. He defended a hostile foreign country while denigrating his own, heaping blame on the United States for the deterioration of its relationship with Russia.

Somehow, in Mr Trump's mind, America had been too mean to Mr Putin.

He made little mention of Russia invading part of a neighbouring country, shooting down a civilian airliner, propping up the Syrian president who gasses his own people, meddling in multiple foreign elections, assassinating inconvenient people around the world or systematically oppressing its own citizens.

What he did mention, with typically tiresome indignation, was the Russia investigation back home, a supposed "witch hunt" that has already secured a number of guilty pleas and just indicted 12 more Russians for hacking Mr Trump's political rivals.

Judging by his performance alongside Mr Putin, and indeed a litany of previous public comments, this massive cyber attack by a foreign power does not concern Mr Trump.

He remains far more agitated by his own intelligence agencies' efforts to discover what happened and deliver justice.

Actually, let's put that even more bluntly.

Mr Trump is desperate to prove he is a legitimate president who was elected without Russia's help. That is what matters to him. He does not care one jot about actually catching the people who tried to subvert America's democracy.

He is putting his own personal interest above his country's. It is not America first, it's Trump first, and in this case the two are mutually exclusive.

Mr Trump's words at the joint press conference with Mr Putin clearly advanced the cause of just one nation - the wrong one. Russia.

He emphasised Mr Putin's "strong and powerful" denial of any election meddling, refusing to stand by the unanimous conclusions of the CIA, FBI and multiple congressional committees.

He didn't press Russia on any of the crimes that have turned it into a global outcast.

Essentially, he gave Mr Putin 46 golden minutes of free propaganda. The proof of that was written all over the Russian president's smug, self-satisfied face.

And all this in return for … what exactly?

It is unclear what Mr Trump was trying to achieve at the summit, beyond a closer personal relationship with Mr Putin. He certainly didn't emerge with anything resembling a win for the United States.

Instead, he made America look weak and subservient to one of the world's most monstrous and immoral tyrants.

The core problem here - and the reason so many on Mr Trump's own side of politics have spent the past two days in full-on freak-out mode - is that Mr Trump can no longer be trusted to protect America's interests.

In other words, he cannot reliably fulfil the most basic duty of his office.

Doubts about Mr Trump's loyalty have lingered for ages, but before this week, they belonged in the realm of wild conspiracy theories.

As the man himself likes to say at every opportunity, there is no evidence he personally colluded with Russia during the election.

But I'm not talking about collusion at the moment, or treason, or anything quite so sinister.

The question is not whether Mr Trump is a secret Russian asset - it is whether Americans can rely on him to selflessly put their wellbeing ahead of his own.

He flunked that test with Vladimir Putin this week. He did not put America first. And if he isn't careful, that failure will haunt the rest of his presidency.